Heretofore, there has been primarily used a solid-state laser such as an Nd:YAG laser or an Nd:YVO4 laser, as a laser light source for generating light of a wavelength band of 1 μm. There have been realized a laser processing machine incorporated with the solid-state laser, and a visible light source using light from the solid-state laser as a fundamental wave. However, in case of using a solid-state laser, it is necessary to cool a laser medium, as the output of the solid-state laser is increased, which may resultantly increase the size of the solid-state laser. In view of the above, there has been a demand for a fiber laser light source capable of outputting light of a Watt-class high-output power with a simplified cooling mechanism for the purpose of welding or to obtain a fundamental wave used at a wavelength conversion light source.
Now, a basic laser operation of a fiber laser light source is described. First, pump light from a pump laser light source is entered through an end of a fiber. The incident pump light is absorbed by a laser active material contained in the fiber to thereby generate seed light of a fundamental wave inside the fiber. The seed light of a fundamental wave is repeatedly reflected and reciprocated in a resonator, wherein a fiber grating formed in the fiber, and a fiber grating formed in another fiber constitute a pair of reflection mirrors. Simultaneously with the above operation, a fiber laser is oscillated by amplifying the seed light with a gain derived from the laser active material contained in the fiber to increase the light intensity, followed by wavelength selection. The two fibers are connected to each other by a connecting portion, and the pump laser light source is current-driven by a pump laser current source.
A part of the output light is separated by a beam splitter, received by a light receiving element for monitoring output light, and then converted into an electrical signal for use. An output controller adjusts a drive current for the pump laser light source using a pump laser current source in such a manner that the light intensity is sufficiently increased to obtain an intended output, based on the intensity of the converted electrical signal. Thereby, the intensity of pump light from the pump laser light source is adjusted, and the output intensity of a fundamental wave from the fiber laser is adjusted. This arrangement enables to perform an automatic power control (hereinafter, called as “APC”) operation of keeping the intensity of an output from the wavelength conversion device at a constant level.
Configuring a laser light source into a pulse light source having a high peak power expands the usage such as laser processing e.g. boring, and high-efficiency wavelength conversion. However, since a currently available light source having a high peak power is of a continuous oscillation type, the purpose of use is limited to e.g. laser welding. A configuration for amplifying modulated seed light from a seed light source by a fiber amplifier has been primarily used as a configuration for emitting pulse light from a light source incorporated with a fiber.
In addition to the above, in the case where a harmonic is generated from a wavelength conversion device based on a fundamental wave generated from a fiber laser, the conversion efficiency from a fundamental wave to a harmonic can be enhanced by generating pulse-like fundamental wave light having a high peak power, rather than subjecting continuous light to wavelength conversion, if the continuous light and the fundamental wave light have substantially the same average outputs. In other words, emitting pulse light from a fiber laser greatly contributes to enhancing the wavelength conversion efficiency.
The idea of emitting pulse light from a fiber laser has also been studied in the communications field. Specifically, patent literature 1 describes a method, wherein a main resonator and a sub resonator are provided, a light modulator is provided in the resonator, and pulse light is generated by matching the beat phases of the main resonator and the sub resonator by the light modulator. Further, there are disclosed a method (see patent literature 2), wherein narrow-band pulse light is generated by inputting pulse light of a high intensity into an optical fiber having an anomalous dispersion characteristic, and using a frequency shift effect; and a method (see patent literature 3), wherein a saturated absorption effect is imparted to a fiber grating portion of a fiber laser resonator.
In the conventional methods disclosed in patent literatures 1 and 2, it is possible to generate ultra narrow-band pulse light. However, the conventional methods involve a problem that the conversion efficiency of the light source may be deteriorated, because it is necessary to provide a modulator in a resonator, or the excitation efficiency is lowered. Even if a saturated absorption band is provided in a resonator, as disclosed in patent literature 3, the conversion efficiency may be lowered, because light loss in the resonator is increased. Further, in all of the arrangements disclosed in patent literatures 1 through 3, a part is required in addition to the elements constituting a fiber laser resonator for generating continuous light, which may increase the cost.
Patent literature 1: Japanese Patent No. 2577785
Patent literature 2: JP Hei 8-146474A
Patent literature 3: JP 2005-174993A